For 4340 steel, the fracture toughness and yield strength depend on the prior heat treatment, as shown in Figure. As yield strength is increased, the fracture toughness decreases. A pipeline is to be built of this steel and to minimize the wall thickness of the pipe, the stress in the walls should be as high as possible without either fracture or yielding. Inspection techniques ensure that there are no cracks longer than 2 mm (a = 1 mm). What level of yield strength should be speci?ed? Assume a geometric constant of f = 1.15.